1. Field of the Invention
The present invention relates to a multi-domain liquid crystal display and a method for manufacturing the same. In particular, the present invention relates to a multi-domain liquid crystal display and a method for manufacturing the same, the liquid crystal display including two opposed substrates (a first substrate and a second substrate) and liquid crystal injected and sealed between the two substrates.
2. Discussion of the Related Art
As shown in FIG. 1, a liquid crystal display is comprised of two opposed substrates (a first substrate and a second substrate), and a liquid crystal is inserted and sealed between the substrates.
The first substrate includes a polarizing plate 1, a transparent substrate 2, a color filter 3, a common electrode 4 and an alignment film 5; and the second substrate includes a polarizing plate 6, a transparent substrate 7, a thin film transistor (hereinafter referred to as TFT) array 8 and an alignment film 9.
The TFT array 8 on the transparent substrate is comprised of a plurality of pixels arrayed in perpendicular and horizontal directions, each pixel having a TFT and a pixel electrode connected to the TFT as a basic unit. The pixels are regions defined by gate bus lines and data bus lines. These bus lines are connected to the corresponding TFTs in the respective pixels.
The color filter 3 is a transparent substrate including one pigment from the group of Red, Green and Blue, and a plurality of color filters are formed in correspondence with each pixel. The common electrode, which generates an electric field relative to the pixel electrode, is formed on the surface of the color filter, and the alignment film is formed on the common electrode.
The liquid crystal 10 is injected between the first substrate and the second substrate, and thereafter the injected liquid crystal is sealed. Also, a spacer 11 is interposed between the two substrates to maintain a constant cell gap.
The liquid crystal display having the construction described above applies a voltage to each pixel on the TFT substrate in order to generate a voltage difference between the pixel electrode corresponding to the pixel and the common electrode on the color filter for rearranging the liquid crystal in that area to display an intended picture image.
The liquid crystal display uses respective color filters corresponding to the three primary colors of light, namely, Red (R), Green (G) and Blue (B). The RBG color filters are positioned close to one another, and a black matrix (BM) (not shown) is formed between color filters situated in adjacent pixels. The black matrix controls the brightness of light when a corresponding signal is applied to the pixel to express the colors.
The TFT liquid crystal display is characterized by a high brightness, a high contrast and low energy consumption, and thus it has been widely applied to desktop computer monitors, notebook computer monitors, TV image receivers, vehicle TV image receivers and navigators, and so forth.
Recently, more advanced liquid crystal displays are in demand in order to keep up with the multimedia era. As an attempt to meet the recent demand, several wide viewing angle (VA) technologies were proposed. However, those technologies have various disadvantages, such as an increase in energy consumption due to a deteriorated aperture ratio, degradation of a front display quality and undesirable effects on the production process.
One of the technologies for securing a wide viewing angle offers an alignment structure of liquid crystal molecules and suggests a multi-domain cell having regions with different structures in a pixel.
For example, an alignment film formed on a substrate undergoes a rubbing treatment or a mechanical treatment to make an alignment treatment in one direction, and a slit pattern is formed in a pixel electrode (or common electrode) on the other substrate to produce the fringe field effect (FFE), thereby providing a pixel with two domains with two different liquid crystal alignment directions. In another case, an alignment film is coated with a photosensitive material, and ultraviolet (UV) is radiated thereon, creating two domains with different alignment directions of the liquid crystal.
FIG. 2 is a plan view illustrating a liquid crystal display having two domains according to the related art. Here, reference numeral 20 indicates a slit-patterned pixel electrode, reference numeral 21 indicates the alignment direction, and a and b indicate average liquid crystal alignment directions at the respective positions when a maximum voltage is applied.
The 2-domain liquid crystal display shown in FIG. 2 is manufactured as follows. First, the alignment film formed on the first substrate undergoes either a rubbing treatment or mechanical treatment in one direction. The second substrate does not undergo the alignment treatment, and the slit pattern is instead formed on the pixel electrode in order to produce the FFE.
Then, the liquid crystal having a negative dielectric constant is injected and sealed between the first substrate and the second substrate. That is, the rubbing treatment determines a pretilt angle for the first substrate, and each pixel in the second substrate has two domains with two different alignment directions a and b due to the fringe field produced by the slit pattern formed in the pixel electrode.
However, the aforementioned method is limited to the 2-domain liquid display only, and is not appropriate for embodying more than two domains since the rubbing treatment may cause various problems.
For example, dust particles and electrostatic charges can be generated during the rubbing treatment, and this may cause severe damage to the liquid crystal display element. In addition, microgrooves created during rubbing are a main factor resulting in light scattering and chaotic phase distortion.
On the other hand, a photoalignment method of irradiating polarized ultraviolet to photosensitive polymers has been already known. According to this method, the alignment ability of photosensitive material is determined by anisotropic material derived from the irradiation. Examples of usable photoalignment materials include polyvinylcinnamate (PVCN), polysiloxane (PS) and polyimide (PI). The photoalignment characteristics of these materials are manifested upon application of UV irradiation.
It is generally believed that the alignment direction of the surface of the alignment material is always perpendicular to the polarization direction of the UV. However, some material is aligned parallel to the polarization direction of the UV.
Compared to the rubbing method in general, the photoaligmnent technology has many advantages since it does not generate electrostatic charges and dust particles, which are typically generated in the rubbing method.
Especially, the photoalignment technology is applicable to the manufacture of 2-domain structure in order to improve viewing angles of the liquid crystal display, optical storage devices, and process devices.
As one example, in the method suggested by W. Gibbons et al (Nature 351 (1991) 49), the photoalignment material is rubbed in a single direction, and then the substrate is irradiated by UV radiation through a mask in order to make the alignment direction perpendicular to the initial rubbing direction.
According to this method, the liquid crystal display is comprised of one substrate having two domains, the other substrate having a single domain that is aligned to the initial rubbing direction, and liquid crystal injected between the two substrates. The liquid crystal cell of this liquid crystal display element has a 90° twisted structure in a region corresponding to a transparent portion in the photoalignment process.
Unfortunately, the above method utilizes the rubbing method of polymers, which generates dust and electrostatic charges upon formation of microgrooves on the surface of the alignment film, leaving the aforementioned problems unsolved.
Another study done by P. Shennon et al (Nature 368 (1994) 532) employs an irradiation method by using polarized light for the initial alignment instead of rubbing the surface of the alignment film.
Although this method successfully solved the problems of the rubbing process, it requires several extra processes since-the alignment film must be exposed to polarized light twice with different polarization directions in order to provide different alignment directions in the respective domains, separately.
Therefore, a new technology needs to be developed to efficiently and effectively construct a multi-domain liquid crystal display.